This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. This Phase II clinical trial sponsored by the Food and Drug Administration will examine the potential therapeutic role for rosiglitazone in 15 patients with newly diagnosed Cushing's Disease, in 15 patients who still have active Cushing's disease despite pituitary surgery, and in 15 patients with non-secreting pituitary tumors either newly diagnosed or who has residual tumor at least 3 months after pituitary surgery. Consenting patients will undergo a detailed baseline historical, physician and biochemical evaluation, including measurement of the specific hormone(s), their pituitary tumor produces, and an MRI assessment of the pituitary tumor size. Patients will commence treatment with Rosiglitazone, (4 mg for i week and then 8 mg by mouth daily for the duration of the study treatment period) approved for use in diabetes mellitus. Biochemical and endocrine examination will be re-evaluated monthly, radiological examination at 6 months, and rosiglitazone therapy will continue for a total treatment period of 6 weeks in the newly diagnosed Cushing's patients (Aim 1), 6 months in Cushing's patients who had elevated cortisol levels after pituitary surgery and 12 months in patients with non-secreting tumors. Patient response will be determined by a statistical comparison of the follow-up assessments with baseline result. A complete response will be defined as normalization of urinary cortisol levels in the Cushing's patients and disappearance of pituitary tumors in the non-secreting tumors from baseline.